1. Field of the Invention
The present invention relates to chemical mechanical polishing. More particularly, the present invention relates to controlling the time during which multiple layers on a substrate are chemically mechanically polished.
2. Description of the Related Art
Endpoint detecting (EPD) and closed loop control (CLC) have each been used individually to establish the point in time at which the chemical mechanical polishing (CMP) of a target layer on substrate is terminated.
There are several techniques that are used for EPD, such as an optical technique, a motor current control technique (in which variations in the motor current caused by the friction produced by different layers are detected), and a non-contact current technique.
As an example of a CMP system employing the optical endpoint detecting technique, the AMAT Mirra CMP system of Applied Materials, Inc. includes a light source that illuminates the target layer, a detector, and a mirror which directs beams of light reflected from the layer being polished to the detector as an optical signal. The detector converts the optical signal to an electrical signal (endpoint detection signal), and monitors for changes in the signal which occur when the polishing process progresses to a layer underlying the target layer. Hence, the endpoint of the chemical mechanical polishing process is detected. With respect to this technique, the intensity of the beams of light reflected from the layers is proportional to the reflectance of the layers. Accordingly, the above-described technique is well-suited for the chemical mechanical polishing of a metal layer, i.e., a layer having a high reflectance.
When multiple layers are chemically-mechanically polished using the optical endpoint detecting method, there are as many variations in the electric endpoint detection signal as there are layers. Accordingly, the processing algorithm is complex and it is difficult to implement a suitable control method.
FIG. 1 illustrates a relationship between an endpoint detection signal (ISRM) and layers being polished when the AMAT Mirra CMP system is used to form a shallow trench isolation (STI) structure on a wafer. As this figure shows, an error in the control occurs during a polishing period xcex94T. The period xcex94T elapses between the time T at which the endpoint of a layer (e.g., an SiN layer in the STI structure) is detected and an endpoint detection position of a practical layer. To deal with this error, the wafers must be checked by lot or sheet unit before they are subsequently processed.
The motor current control method also suffers from the above-described problems and limitations. After one layer of a 2-layered structure is polished, the polishing of the other layer can be performed only by time control when using the motor current control method, similar to the optical endpoint detecting method. Accordingly, the motor current control method cannot cope with changes in the polishing rate and variations in the thickness of the other layer of the 2-layered structure.
The closed loop control endpoint detection method is a type of trial-and-error method in which subsequent processes are controlled according to information collected from the previous processes. The closed loop control method can be classified as a sheet unit control method or a lot unit control method. A polishing process using the closed loop control method will now be described with reference to FIG. 2.
First (step S20), a CLC manager measures pre-polished thickness data (pre TOX) using a corresponding lot of wafers and stores the data in a database (not shown). Next (step S30), the CLC manager calculates the polishing time necessary to attain the optimized polishing thickness. The CMP process is carried out for a duration corresponding to the calculated polishing time (step S40). Post-polishing thickness data (post TOX) is then measured (step S50), and the polishing rate is calculated using the pre TOX and the post TOX. In the lot unit wafer process, the polishing time for a current CMP process is set by dividing a difference between the pre TOX and the target value by the polishing rate calculated at the end of the previous CMP process.
Also, the measured thickness data is compared to process specifications (step S60). If the measured thickness data deviates from the process specifications, a statistical process is preformed during which time a pause in the CMP process takes place (step S70). In this case, an operator must take immediate action, such as changing the polishing pad. The polishing time is recalculated after the polishing pad is changed or the problem is otherwise dealt with.
The above-described CLC method can be readily applied to a CMP process for polishing a single layer. However, it is difficult to apply the CLC method to a CMP process for polishing a 2-layered structure, such as an STI structure, because the polishing rate differs among the layers constituting the 2-layered structure.
Accordingly, the chemical mechanical polishing of a 2-layered structure necessitates a complex control method and lots of processing time. As previously described, it is difficult to execute a satisfactory polishing time control when a 2-layered structure is polished by a conventional CMP system relying on an optical endpoint detection method. Therefore, samples must be constantly checked, i.e., additional processes must be employed. The closed loop control method can not be used exclusively in a CMP process for polishing a 2-layered structure, such as an STI, because the polishing rates of the two layers constituting the 2-layered structure are different from each other.
An object of the present invention is to provide a polishing system and method which can effectively control the times during which different layers of a multi-layered structure are polished.
According to one aspect of the present invention, a system for controlling the polishing times of different layers on a semiconductor wafer includes a chemical mechanical polishing (CMP) apparatus, a measuring apparatus for measuring pre-polishing thickness and post-polishing thickness of the layers polished by the CMP apparatus, a database for storing the pre-polishing thickness and post-polishing thicknesses that are measured by the measuring apparatus, an endpoint detection system for controlling the CMP apparatus to operate in an endpoint detection mode, a closed loop control (CLC) manager for receiving the thickness data from the measuring apparatus, calculating a polishing time based on the thickness data, and controlling the CMP apparatus to operate in a closed loop control mode, and an operator interface by which the CMP apparatus is selectively operated in the endpoint detection mode and the closed loop control mode.
For instance, the operator interface allows an operator to input the recipes of the polishing processes carried out under the endpoint detection and closed loop control modes. The operator interface may also or alternatively allow the operator to monitor the progression of the polishing process.
The system may polish semiconductor wafers by lot or by sheet unit. The endpoint detection system may be an optical endpoint detection system or a motor current control feedback system.
In any case, once the CMP apparatus completes the polishing of an upper layer as confirmed by the endpoint detection system, the operator interface receives control information from the CLC manager and automatically inputs a recipe containing a set value, i.e., a polishing time, by which the CMP apparatus polishes the lower layer under the control of the CLC manager.
Therefore, according to another aspect of the invention, a process for polishing a multi-layered structure basically includes a first step of preparing process recipes for the polishing of upper and lower target layers of the structure, a second step of polishing the upper target layer using an endpoint detecting technique, and a third step of polishing the lower target layer using a closed loop control technique.
Thus, the substrate is monitored while the upper target layer is being polished. Once the upper layer is completely polished as confirmed by endpoint detection (the detection of the state in which the lower target layer is exposed), the thickness of the lower target layer is measured. A polishing time is calculated based on the measured thickness of the lower target layer. The lower target layer is then polished according to the calculated polishing time. Specifically, the polishing time is calculated using empirical data derived from each time the CMP apparatus is used to polish a lower target layer. In this case, the empirical data is preferably weighted according to the sequence in which it is derived.